The present invention relates to the removal of ferric iron and other impurities from copper electrolyte solution used in the electro-winning of copper or other acidic aqueous solutions containing metal ions.
Copper metal is produced from copper-bearing ore using several well-known processes. One of the most commonly used processes is referred to as heap leaching. In this process dilute sulfuric acid is passed over copper-bearing ore that has been placed atop an impermeable liner. As the dilute sulfuric acid solution percolates through the copper-bearing ore, copper and other impurities are leached from the ore. This solution, referred to a pregnant leach solution or PLS, is collected atop the impervious liner and is routed to a processing facility to recover the copper and return a copper-depleted solution for subsequent leaching.
Copper is recovered from the PLS using a multi-step process called solvent extraction and electro-winning (SX-EW). In the first step, copper ions are extracted from PLS using a kerosene-type solvent mixture. The copper depleted PLS is then returned to the heap leach for additional leaching. Copper is then stripped from the solvent mixture using a copper sulfate-sulfuric acid mixture (CuSO4xe2x80x94H2SO4). The final step of the process is electroplating copper from the copper-enriched solution of the copper sulfate-sulfuric acid mixture.
Small amounts of iron are commonly transferred with the copper to the electroplating solution. Iron is either chemically co-extracted with the solvent or is mechanically introduced as entrainment of the aqueous solution from the stripping. As copper is plated out of solution, the iron content in the electrolyte increases. The build-up of iron in the electro-winning solution results in a decrease in the current efficiency due to continuous oxidation and reduction of iron (Fe++/Fe+++). Operating conditions of the copper electro-winning circuit are such that iron cannot be reduced to metal at the cathode; hence iron remains in the system. The loss of current efficiency can amount to 1-3% per gram of iron per liter of electrolyte. Normal plant practice to control iron in the electrolyte is to occasionally bleed iron rich copper electrolyte and replace it with sulfuric acid electrolyte.
The copper electro-winning process uses lead-alloy anodes. Soluble cobalt is added to the electrolyte (40-250 ppm) to minimize corrosion of the anodes and to prevent lead contamination of the copper metal plated at cathode. During the bleed of the electrolyte to control iron, cobalt is therefore lost and fresh cobalt must be added to the electro-winning electrolyte to maintain the needed level to minimize corrosion of the anode. Replenishing of cobalt to the electrolyte is a major operating expense in copper SX-EW operations. A method of removing iron from electro-winning electrolyte without removing cobalt has long been desired.
The following patents provide proposals for the selective removal of iron from an acidic solution; but none has provided a satisfactory method which is suitable for use in the removal of impurities from a process of electro-winning of copper.
Gula et al in U.S. Pat. No. 5,582,737 describe a process that selectively separates iron (III) from sulfuric acid solutions containing copper and cobalt using an ion exchange resin that contain gem-diphosphonic functional groups. The resin preferentially adsorbs iron (III) and copper and cobalt are returned to the electroplating solution. Loaded resin is regenerated using sulfurous acid (H2SO3) generated by aspirating SO2 gas into water containing copper (I) ions. Sulfurous acid reduces the adsorbed iron (III) ions to iron (II) that are easily removed and discarded as a waste product.
Cameron in U.S. Pat. No. 5,603,839, describes a process for the recovery of waste sulfuric acid generated at industrial operations. An ion exchange resin is used to separate sulfuric acid from waste sulfuric acid. The patent teaches that the resin is regenerated with water to produce a acid-rich solution and a salt-rich solution. The acid-rich solution is then further concentrated with a multi-step evaporator to produce a concentrated acid-rich stream for recycle.
Dreisinger et al. in U.S. Pat. No. 5,948,264 describe an improved gem-diphosphonic resin regeneration process for the removal of iron (III) from aqueous metal ion containing sulfuric acid solutions. The patent teaches that increasing the temperature of the sulfurous acid to 65-95xc2x0 C. improves the regeneration of the ion exchange media.
Bauman et al in U.S. Pat. No. 2,738,322 describe a process for removing sulfuric acid from aqueous solutions of inorganic sulfates using anionic exchange resin. They teach that any anionic exchange resin containing primary, secondary, or tertiary amino groups or quaternary ammonium groups may be employed in the process. Aqueous solutions of inorganic sulfates containing sulfuric acid are passed through a bed of anion exchange resin. Sulfuric acid is retained on the resin and the inorganic sulfate salts pass through. Sulfuric acid is then recovered from the anion exchange resin by washing the resin with water. The process can be operated at room temperature.
Alexander et al, U.S. Pat. No. 6,232,353, teach that by synthesizing an ion exchange resin with both sulfonic and diphosphonic acid functional groups the selectivity for iron (III) is improved over transition metal. Iron loaded resin. However, the resin must be regenerated with hot sulfuric acid solution containing Cu(I) solution. This is an improvement in the selectivity of the resin over the diphosphonic resin described by Dressinger, et al, U.S. Pat. No. 5,948,264.
It is an object of the present invention to provide a method for electro-winning of copper n which impurities and particularly iron is removed.
According to the invention there is provided a method for the electro-winning of copper comprising:
providing electrodes in an electrolyte comprising a copper-enriched aqueous solution of a copper sulfate-sulfuric acid mixture containing iron (III) as an impurity and passing current therethrough to effect extraction of copper by electroplating of the copper from the electrolyte;
the extraction of the copper causing a relative build up in the level of the iron impurity in the electrolyte;
after extracting copper, removing at least a part of the electrolyte solution;
removing the iron (III) impurity from the aqueous sulfuric acid electrolyte solution containing the iron impurity by:
contacting the removed electrolyte solution with an anionic strong base solid ion exchange medium in which the medium binds the sulfuric acid in preference to the iron (III) ions to form a solid/liquid admixture, wherein the ion exchange medium is comprised of an insoluble cross-linked polymer having a plurality of quaternary amine groups (N::) as the exchanging group;
contacting the electrolyte solution with a sufficient amount of ion exchange particles for a time period required to solid phase-bind the sulfuric acid to leave an aqueous phase containing the iron impurity;
separating the solid and liquid phase;
extracting the iron impurity from the liquid phase;
removing the sulfuric acid from the solid ion exchange medium;
and returning at least part of the sulfuric acid and the liquid phase after extraction of the iron impurity to the electrolyte solution for further electrolytic plating of the copper.
Preferably the liquid phase is mixed an acid-neutralizing agent to raise the pH of the liquid phase to a value of at least 2.5 but less than 4.0, causing the iron (III) to form a solid phase iron hydroxide (Fe(OH)3) for separation therefrom.
Preferably the solid phase iron hydroxide is then separated from the liquid phase and the liquid phase is returned to the electrolyte solution for further copper extraction.
Preferably the acid neutralizing agent is sodium hydroxide solution.
Preferably the sulfuric acid is removed from the ion exchange medium by contacting the solid phase-bound sulfuric acid with water thereby forming an aqueous sulfuric acid solution and regenerating the ion exchange medium.
Preferably the aqueous sulfuric acid solution separated from the ion exchange resin is recycled back to the electrolyte solution.
Preferably the solid ion exchange medium is provided in the form of particles.
Preferably the solid ion exchange medium is comprised of a cross-linked copolymer ranging from 4% cross linked to 40% cross linked.
Preferably the solid ion exchange medium is comprised of a cross-linked copolymer with the preferred cross linking greater than 8%.
Preferably the solid ion exchange medium is comprised of a cross-linked copolymer present as spherical particles with a diameter of between 0.074 millimeters and 1.0 millimeters.
Preferably the solid ion exchange medium is comprised of a cross-linked copolymer with the preferred particle size less than 0.297 millimeters.
Preferably the concentration of sulfuric acid in the removed electrolyte solution is about 1 to 3 molar.
Preferably the removed electrolytic solution contains other metal ions from the group consisting of copper (II), manganese (II), cobalt (II) and iron (III).
Preferably the said aqueous solution of sulfuric acid forming the removed electrolytic solution also contains iron (II) and iron (III) ions.
Preferably the solid ion exchange medium is provided in a moving bed.
Preferably the electrolyte solution contains cobalt and the cobalt extracted with the removed electrolyte solution is returned to the electrolyte solution after the iron impurities are removed.
The present invention relates to an improved method for removing iron (III) from aqueous metal ion-containing sulfuric acid solutions using anionic strong base (SBR) ion exchange resin containing quaternary ammonium groups (N::) as the exchanging group.
Suitable strong base ion exchange resins are described in detail in United States patents set out hereinafter.
An improved ion exchange process is disclosed herein that avoids the use of gem-diphosphonic acid ion exchange resin and thereby negates the need for regeneration with copper (I) ions and the handling of sulfurous acid and SO2. The process is based on the use of strong base resin (SBR) that has the ability to separate acid from mixtures of acids and salts such as the copper electrolyte solution.
An aqueous metal ion-containing sulfuric acid electrolyte solution that contains iron (III) ions as well as at least copper metal ion is contacted with solid ion exchange medium that is preferably in the form of particles. The ion exchange medium binds sulfuric acid in preference to the additional metal ions present to form a solid liquid admixture.
The preferred ion exchange medium is comprised of cross-linked copolymer ranging from 4% cross linked to 40% cross linked with the preferred cross linking greater than 8%, preferably present as spherical particles with a diameter of between 0.074 millimeters and 1.0 millimeters with the preferred particle size less than 0.297 millimeters and having a plurality of quaternary ammonium groups.
The contact is maintained between the sulfuric acid solution containing iron (III) ions and a sufficient amount of ion exchange particles for a time period required to solid phase-bound sulfuric acid and an aqueous phase containing the additional metal ions.
The solid and liquid phases are separated.
In one embodiment, the liquid phase containing iron (III) and the additional metals is contacted with a small amount of sodium hydroxide or other base to raise the pH above pH 2.5 preferably pH 3 causing the iron (III) to precipitate as ferric hydroxide (Fe(OH)3) solid.
In another embodiment of the invention, hydrogen peroxide is added to the liquid phase and iron (II) is allowed to oxidize and hydrolyze as ferric hydroxide. In yet another embodiment, the acid-free solution of iron (III) and additional ions is returned to the heap leach process.
In another embodiment, the oxidation potential and the pH of the liquid phase are raised to precipitate manganese.
The ferric hydroxide solid is separated from the acid-free solution containing the additional metals using a filter. The solid ferric hydroxide is washed with water to recover entrained metal and the wash solution as well as the filtrate is returned to the process.
The ion exchange medium containing the bound sulfuric acid is rinsed with sufficient water, the preferred method being countercurrent, for a time period to form an admixture of solid phase-bound water and sulfuric acid.
The solid and liquid phases are separated.
The liquid phase containing sulfuric acid is returned to the process and solid phase defining the ion exchange particles is ready to be reused as described above.